The prior art has developed a variety of linear flowmeters in which a manometer or other device for measuring a pressure differential is connected across opposite sides of a flow restrictor. The restrictor comprises one or more passageways proportioned so that under normal working conditions the resistance to flow through the resistor as a whole is substantially proportional to the rate of flow. There are certain levels of inaccuracies inherent in such devices and methods have been developed to optimize the accuracy of results obtained. See for example Goldsmith U.S. Pat. No. 3,071,001 and Weichbrod U.S. Pat. No. 3,071,160. See also application Ser. No. 141,897, now U.S. Pat. No. 3,792,609 entitled Flow Splitter by R. F. Blair, R. J. Hill and D. B. Le May of common assignment to the present application.
In certain applications the flow rate of a fluid is measured not by directly determining the pressure differential across a restrictor, but by measuring the actual flow of a small portion of fluid. Such applications require that the flow of the fluid be divided into two or more paths with an exact ratio maintained between the individual path flow rates. In a typical situation, such as in a mass flowmeter, a very small percentage of the flow is diverted into a measuring section. This percentage may be as small as 1 part in 40,000 and the flow measuring section is typically a very thin tubular conduit which is much longer that its diameter so that laminar flow prevails throughout the conduit. During laminar flow of a fluid, the flow rate is directly proportional to pressure drop and inversely proportional to viscosity. In contrast, during turbulent flow, the flow rate is proportional to the square root of pressure drop and largely independent of viscosity. Therefore, in the design of a flowmeter in which the flow is split along parallel paths, it is important to provide conditions that will assure laminar flow in each path. Since the measuring section flow is laminar, if the bypass flow were turbulent the flow ratio would be a function of viscosity and would have an undesirable dependency upon temperature and pressure. Such flow splitters are thus much more prone to inaccuracies as a result of geometric configuration than are pressure differential devices.
In the above-mentioned application Ser. No. 141,897 a number of devices are disclosed incorporating a plurality of closely spaced fluid passageways, e.g., defined by a plurality of screens, each passageway defining a laminar flow path. Generally, improvements over prior devices is thus obtained, but the pressure drop is influenced by the compressive force used to assemble the device and by variations in screen mesh. The effective diameters are quite small, which can result in trapping of contaminants and resulting plugging.
The present invention provides simple and economical methods for assuring laminar flow in both the measuring section and bypass section of a flow splitter so that a constant and predetermined ratio is maintained across the entire range of flow rates to be measured. The present construction overcomes the disadvantages referred to above and additionally provides a wide range of flow, as high as 1,000:1 or higher, obtained with facility and repeatable accuracy. This has been accomplished by using as a flow restrictor one or a juxtaposed plurality of disks, each having one or a plurality of channels formed from its perimeter to an opening through opposite sides of the disk. Fluid is directed to the perimeter of the disk or disks and is conveyed by the conduits to the opening. The conduits have sufficiently large length to diameter ratios (or length to effective hydraulic radius, as defined hereinafter) to assure laminar flow of the fluid. In an exemplary embodiment, the flow restrictor comprises flat, smooth sided juxtaposed disks, formed with central, aligned openings. The central opening in each disk communicates via one or more conduits radiating from the central opening to the perimeter of the disk.
By juxtaposing a predetermined number of disks having a predetermined number and shape of conduit, one can achieve any flow ratio desired. There is only one flow path, radially inward (or outward if flow is reversed); therefore, the passage size can be precisely controlled. Furthermore, the conduits have sufficient diamter so that plugging by contaminants can be avoided. The entire construction can be made of metal and can be tightly assembled so that seals are not required, enabling its use with otherwise corrosive fluids.
The flow restrictor is combined with an elongate laminar flow conduit, serving as a measuring section, to form a substantially linear flowmeter. Such a meter includes a housing having a fluid inlet and fluid outlet, the housing defining a fluid path between the inlet and outlet. The flow restrictor is disposed in this fluid path in parallel circuit with the measuring section conduit. Means are provided for measuring the rate of flow of fluid through the measuring section conduit, which means are known in the prior art and constitute no part of the present invention. The result is a compact structure of simple construction which demonstrates high accuracy in measurements over a substantial range of flow temperature and pressure conditions.